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technology  GaN


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here is no question that GaN LEDs and lasers are a great success. They backlit billions of screens, they lie at the heart of countless Blu-ray players, and they are driving a revolution in energy-efficient lighting. However, that is not to say that these devices are without fault. In fact, they have several downsides, including internal electric fields that pull apart the electrons and holes in the quantum wells, hampering light emission (see Figure 1).


Separation of the carriers by internal fields, which is referred to as the quantum confined Stark effect, has impeded the development of conventional GaN-based green lasers that could be deployed in red-green-blue laser displays. Producing nitride devices that emit at this wavelength requires indium-rich InGaN quantum wells, but the greater the indium content, the stronger the internal electric fields that pull the carriers apart.


Internal fields are also bad news for LEDs. One of the weaknesses of this device is droop, a reduction in light-emitting efficiency at higher drive currents. The origin of this mysterious malady is hotly debated, but its two most popular explanations – Auger recombination and a spilling over of electrons from the quantum well – suggest that internal electric fields are detrimental. That’s because these fields increase the likelihood that electrons will spill over into the hole-emitting region of an LED; and they also prevent efficient operation of devices with wide wells, which enable a reduction in carrier density and lower Auger recombination rates.


To overcome the problems associated with these electric fields, some researchers have switched from conventional substrates to those that are described as semi-polar or non-polar. Growing devices on alternative platforms either reduces substantially or eliminates the internal electric fields in the device (see Figure 2). Milestones in the advancement of such devices include: The first reports of non-polar lasers in 2007, independently developed by Rohm and the University of California, Santa Barbara; semi-polar green lasers fabricated by a partnership between Sony and Sumitomo Electric that emit a continuous output above 100 mW at wavelengths longer than 530 nm; and non-polar LEDs announced by Panasonic at the International Electron Devices Meeting that deliver a light output efficiency of almost 40 percent at a current density of 1 kA cm-2


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These performances highlight the benefits that result from moving to growing devices on semi-polar and non-polar substrates. However, switching to these novel planes pays a heavy price: A hike in the cost of the substrate, which stems from the difficulties associated with making it. GaN substrates that provide a platform for the growth of c-plane devices, mainly lasers, have been available for several years, and prices are falling, with 2-inch material now costing around $1000. These substrates tend to be made by a HVPE process, leading to the deposition of a relatively


Semi-polar planes are promising orientations for making green-emitting structures


thick layer of GaN on a foreign substrate, such as sapphire or GaAs. The wide bandgap crystal is subsequently removed and sliced into wafers. Cutting perpendicular to the growth direction yields c-plane substrates, while slicing in other directions produces semi-polar or non-polar material.


The downside of this approach is that because it is not easy to grow a very thick GaN crystal, the sizes of the semi-polar and non-polar substrates that are sliced from it are limited. They are typically just 10 mm by 20 mm in size, and sometimes just 10 mm by 10 mm, and they retail for around $1000. In addition to the high cost of the real estate on these planes of GaN, their small sizes are incompatible with wafer processing lines. Together, this pair of weaknesses forms a major barrier to the commercial progress of non-polar and semi-polar lasers and LEDs.


Slashing prices


Since 2006, a group of German Universities have been collaborating to try and develop low-cost foundations for the growth of semi-polar and non-polar optoelectronic devices. Their programme, which is named PolarCoN and is co-ordinated by Ferdinand Scholz from Ulm University, won funding from the German Research Foundation in 2008. Now in its second phase, this €4.5 million project involves, in addition to the University of Ulm, seven other universities: Stuttgart University, Otto-von- Guericke University Magdeburg, TU Braunschweig, TU Berlin, Regensburg University, Freiburg University and Kassel University.


“Originally, our main target was the green laser,” admits Scholz, who explains that researchers outside of the project, such as those at Sumitomo, have now succeeded in that endeavour. “We


March 2013 www.compoundsemiconductor.net 45


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